In the past, a torque controlling technique (refer to the Patent Publication Gazett Tokukaihei 6-42789) is known which controls an input voltage or an input current of an inverter motor so that vibration following the change in rotation speed within one rotation of a compressor having one cylinder is decreased.
Further, a brushless DC motor is devotedly employed as a driving source which is easier in torque controlling than an AC motor due to a position detection mechanism which is previously provided to the brushless DC motor.
Among brushless DC motors, when a surface magnet arrangement brushless DC motor, in which a rotor has permanent magnets which are provided at a surface of the rotor, is employed and torque is controlled, a method for controlling a d-axis current to be 0 which gives no influence to motor torque generation, that is the method for controlling a phase of current to be the same phase as a phase of a generation voltage due to a motor speed (the phase of current equals to 0), is known as a driving method which is free from lowering in efficiency, and is widely employed because of simplification in control.
On the other hand, an embedded magnet arrangement brushless DC motor, in which a rotor has permanent magnets which are embedded in an interior of the rotor, can simultaneously output two generation torques, that is a magnetic torque and a reluctance torque. Therefore, the embedded magnet arrangement brushless DC motor has a characteristic effect such that driving with higher efficiency than the surface magnet arrangement brushless DC motor is realized by determining distribution of the two torques to be adequate in response to a load torque so that the current is determined to have a minimum value and the total torque is determined to have a maximum value (hereinafter, referred to as "maximum torque control"). The embedded magnet arrangement brushless DC motor has developed in application to an air-conditioner or the like in recent years which is required energy saving especially.
Further, a maximum torque control method for controlling an embedded magnet arrangement brushless DC motor is recited in "a controlling method which is adequate to an embedded magnetic arrangement PM motor (Umekomi-jishaku-kouzou-PM-mota ni tekishita seigyphou)", Motimoyo et al., Denki-gakkai Handoutai Denryoku Kenkyuukai Shiryou SPC-92 5. It is known that the maximum torque control is -realized by controlling d-, and q-axis currents based upon a relational equation which is determined based upon electrical constants of a motor.
But, when the maximum torque control and the torque control are combined with one another, the following disadvantages arise.
(1) Modeling errors due to a motor temperature and magnetic saturation-are generated so that a maximum torque condition is not satisfied constantly. And, for solving problems (specifically, changes in winding resistance and the speed electromotive force constant following an increase in temperature, and changes in d-, and q-axis inductance values and the speed electromotive force constant due to magnetic saturations due to modeling errors of a motor, changes in various parameters due to temperature and magnetic saturation should be actually measured and are considered in operations. This is extremely difficult in actual application.
(2) When the maximum torque control is combined with torque control which cancels harmonic components up to higher harmonics which gives little influence in vibration, electric power is consumed beyond the necessary amount so that driving with high efficiency is not realized.
(3) A peak current is increased by the torque control so that the peak current is over the limit value of an inverter current. Therefore, an operation point should be shifted from an operation point of the maximum torque control so that efficiency is decreased accordingly.